C'nor asked what I meant by the term, because different age ranges might mean different ideas of what would be appropriate. Here are some thoughts on the matter.
When I said I wanted to design a kid-friendly adventure I was mostly thinking I would 1) turn my creep meter down, 2) be more careful in my assumptions about how violence and thievery are rewarded, and 3) incorporate toy-like qualities that would be fun to explore.
I think 2 and 3 could just as easily apply to adults, so really the only difference is 1.
Nightmares aren't the Goal
I'm not certain about all this, I mean, I know kids like to be scared. But I think something like the unborn might be too much of nightmare fuel. (I was telling one of my adult players about them over the phone and making the noise they make and she started freaking out, "Ok, ok I don't want to hear about them.")
I'm not even sure I would do this intentionally to an adult. I want to make players uneasy, to feel the game, to be immersed, but not actually be terrified. I had a player who just lost a fighter to a giant spider bite to the face say, "Thanks a lot bastard, you know I'm arachnophobic, right?" I said, "What? No, sorry!"
What are We Actually Saying about the World?
I like a lot of things about B2 but I get uneasy when the main thrust of it is to go exterminate whole tribes of sentient evil things. Call me a sissy hippie, but I just start thinking of Bury my Heart at Wounded Knee. (that's not to say I would never have evil creatures with human-like qualities in a game, but I feel especially leery about underscoring that tendency in young humans).
Also, young players will have plenty of opportunity to experience violence in the real world. It won't hurt them a bit if my world challenges the assumption that violence is the first or only solution to problems.
What the heck do kids like about Pokémon and Bakugon type toys? I think there is a lot of potential for experimenting, combining, collecting, and making-type fun that isn't found in a lot of adventures. Whatever wondrous machine keeps a party so occupied they stay in a room through multiple wandering monster checks, that's the machine I want to design. I don't want this instead of exploration, resource management, adventure fun, I want it in addition.
So, I didn't really answer the question about age range, I think the parent DMs are better suited to address that. But what do you all think about the three goals?